Distributed Brillouin fiber sensors are rapidly being adopted for many applications, including but not limited to, structure health monitory (SHM), geotechnical engineering, power lines, oil and gas pipe lines, and oil drilling. Brillouin-based sensor technology operates in two regimes: (a) stimulated Brillouin scattering (i.e., Brillouin optical time domain analysis, or BOTDA); and (b) spontaneous Brillouin scattering (i.e., Brillouin optical time domain reflectometry, or BOTDR). Both BOTDA and BOTDR regimes utilize the linear dependence of the Brillouin frequency shift on temperature and/or strain of the tested component(s).
One problem with the implementation of distributed Brillouin fiber sensors is the sensitivity of the Brillouin frequency shift (BFS) to both strain and temperature. This effect leads to ambiguity in the measurements. In particular, conventional approaches fail to isolate the change in strain and/or temperature of the tested component(s) associated with an observed BFS.
One approach used to address this problem is the use of two fibers placed adjacent to each other, in which one fiber is isolated from any strain effects. The isolated fiber can be used to monitor the temperature, while the other fiber will measure the effect of both strain and temperature. However, this approach is subject to at least two types of measurement errors. First, the isolated fiber is not totally strain free, which results in measurement errors associated with temperature. Second, the different length of the two fibers from the input to the sensing location results in measurements at two different locations, leading to additional measurement errors.
In another two-fiber approach, two fibers with different Brillouin properties are used to sense both temperature and strain. As such, the BFSs of the two fibers are measured. The temperature and strain levels are calculated based on the coefficients of strain and temperature of the two fibers. Nevertheless, this approach is susceptible to measurement errors according to the second type of error described above, i.e., errors associated with measurements at differing locations on the two fibers.
A one-fiber based method has also been attempted to address BFS-related measurement errors. In particular, a fiber with multiple Brillouin peaks is used as the sensing fiber. The different dependencies of the BFS peaks are used by this approach to discriminate between temperature and strain. However, this method, which depends on an evaluation of multiple BFS peaks of the fiber, leads to poor spatial resolution, limited sensing accuracy, and short sensing distance.
There is therefore a need for a Brillouin fiber sensor system capable of both improved spatial resolution and accurate, simultaneous measurements of temperature and strain.